Lower turnout equals higher uncertainty in French election arithmetic

PARIS (Reuters) - High noon on Sunday will bring the first hard sign of just how close France's presidential race really is with the release of early turnout figures for the first round eight hours before expected results.

With indecision also a major factor, polls show the race is so tight between the top four candidates that each has a chance of making the two-person run-off vote - therefore presenting no fewer than six second round scenarios.

Reuters research into past elections shows that the lower the first-round turnout, the greater the uncertainty about which two candidates will contest that run-off on May 7.

Investors are poring over the arithmetic of past votes for clues about the likelihood of an unexpected result this time.

Judging from history, turnout will be the key variable. In past elections, the higher the abstention rate in the first round of voting, the lower the hurdle candidates had to clear in order to qualify. http://tmsnrt.rs/2p9mLpI

French bond yields have risen in recent months and equity investors have massively hedged positions in the options market to reduce exposure to the risk of a market-unsettling surprise vote.

The Interior Ministry will publish a first turnout estimate at midday (1000 GMT) on Sunday followed by an update at 5:00 PM, three hours before the last polling stations close at 8:00 PM.

Anything lower than rates seen in 2002 when turnout hit record low levels could raise the chances of a surprise result. Turnout was only 21 percent at midday then and 58 percent at 5:00 PM.

Polls have consistently suggested centrist Emmanuel Macron and far right leader Marine Le Pen will score highest in the first round with about 22-24 percent of the vote each - and thereby qualify for the runoff.

But conservative Francois Fillon and hard-left candidate Jean-Luc Melenchon are not far behind, touching 20 percent in some polls, putting both in striking distance of qualifying when taking margins of error into account.

RISK OF SURPRISE

"This (closeness) creates a risk of surprise because it substantially lowers the vote required to make it to the second round," Swiss fund managers Unigestion, with 23 billion euros ($24.5 billion) under management, said in a research note.

In French presidential elections since 1965, the lowest scoring of the two candidates to qualify for the runoffs has had a vote of 25 percent on average.

However, Le Pen's father Jean-Marie, when he was head of the party she took over in 2011, qualified in 2002 with only 16.9 percent of the vote - a record low qualification level on a record abstention rate of 28 percent, proving polls at the time embarrassingly wrong.

And since 1965 there has been an 80 percent negative correlation between the score of the second best performing candidate in the first round and voter abstention, according to a Reuters calculation.

That means the higher the number of people who do not vote, the lower the threshold to qualify will be.

Polls have consistently suggested that the turnout will be low. An Elabe poll on Monday found that only 68 percent of those surveyed were certain to vote.

While Macron vs Le Pen remains most the likely second-round scenario, for the investment community, a positive surprise could be an outcome that pits Macron against Fillon, while a negative one would be Le Pen versus Melenchon.

"A higher perceived risk of a run-off between these two Eurosceptic candidates has already impacted markets," HSBC said in a research note on Tuesday.

Basing their assumptions on decisiveness research, some pollsters and financial analysts say a low turnout favors Le Pen, whose supporters show up as the most sure about their vote and the most determined to vote.

However this is not a given, according to Francois Miquet-Marty of pollsters Viavoice, who notes that Le Pen relies heavily on young and working-class voters - two groups where abstention is forecast to be high.